QModbus in a Thread

[hkottmann]

On my programm I have a worker thread (QThread) that is independent from the GUI-thread which is collecting data from an RS232 device and a modbus (RS485) device. While RS232 is easy to handle, I have a problem with the modbus device since QModbus is obviously running in a separate thread. I have programmed it like the modbus example that comes with the QT installation, so I send a read request and connect the reply with a slot which takes care of picking up the data. Currently I'm waiting for the answer using this code:

do
    {
        QEventLoop(this).processEvents();
    } while (!answerReceived);

But this seems to be an ugly hack, so I get tons of such error messages in QTCreator:

QBasicTimer::start: Timers cannot be started from another thread

What would be the proper way to implement this?

[hkottmann]

I've tried these two suggestions from here, but I still get tons of these timer errors:

https://stackoverflow.com/questions/5610785/how-to-tell-qthread-to-wait-until-work-is-done-and-then-finish

[hkottmann]

Here the full code of the involved functions:

void Worker::requestValue(int address, int length)
{
    answerReceived = false;
    qDebug() << "Read values";
    read(address, length);
    do
    {
        QEventLoop(this).processEvents();
    } while (!answerReceived);
}

void Worker::read(int address, int length)
{
    if (!modbusDevice)
        return;
    if (auto *reply = modbusDevice->sendReadRequest(readRequest(address, length), 10)) {
        if (!reply->isFinished())
            connect(reply, &QModbusReply::finished, this, &Worker::onReceiveValues);
        else
            delete reply; // broadcast replies return immediately
    } else {
        qDebug() << tr("Read error: ") + modbusDevice->errorString();
    }
}

void Worker::onReceiveValues()
{
    auto reply = qobject_cast<QModbusReply *>(sender());
    if (!reply)
        return;

if (reply->error() == QModbusDevice::NoError) {
    const QModbusDataUnit unit = reply->result();
    QVector<quint16> receivedValues = unit.values();
    switch(answerType)
    {
    case AnswerType::Flow:
    {
        prominentPumvValues.flow = processFloatValue(receivedValues);
    }
        break;
    case AnswerType::Stroke:
    {
        prominentPumvValues.stroke = processFloatValue(receivedValues);
    }
        break;
    case AnswerType::Frequency:
        prominentPumvValues.frequency = receivedValues.at(0);
        break;
    case AnswerType::SerialNumber:
    {
        prominentPumvValues.serialNumber = processString(receivedValues);
    }
    default:
        break;
    }
} else if (reply->error() == QModbusDevice::ProtocolError) {
    qDebug() << tr("Read response error: %1 (Mobus exception: 0x%2)").
                                arg(reply->errorString()).
                                arg(reply->rawResult().exceptionCode(), -1, 16);
} else {
    qDebug() << tr("Read response error: %1 (code: 0x%2)").
                                arg(reply->errorString()).
                                arg(reply->error(), -1, 16);
}

reply->deleteLater();
answerReceived = true;
}

I've found this thread here: https://forum.qt.io/topic/82819/qbasictimer-start-timers-cannot-be-started-from-another-thread-issue/10 and modified the read function as follows:

void Worker::read(int address, int length)
{
    if (!modbusDevice)
        return;
    if (auto *reply = modbusDevice->sendReadRequest(readRequest(address, length), 10)) {
        if (!reply->isFinished())
        {
            qDebug() << "Reply not finished";
            qDebug() << "Connect: " << connect(reply, &QModbusReply::finished, [reply, this]() {
                qDebug() << "connect";
                if(reply->error() == QModbusClient::NoError)
                {
                    const QModbusDataUnit unit = reply->result();
                    receivedValues = unit.values();
                    qDebug() << "Lambda: " << receivedValues.length();
                }
                else
                    qDebug() << "Reply error";
                reply->deleteLater();
                qDebug() << "Bye";
            });
            qDebug() << "Ade";
        }
        else
        {
            qDebug() << "Reply finished";
            delete reply; // broadcast replies return immediately
        }
    } else {
        qDebug() << tr("Read error: ") + modbusDevice->errorString();
    }
    qDebug() <<receivedValues[0] << receivedValues[1];
}

The answer of connect is true but so it looks the code inside the lambda is not executed, there is no output of the qDebug() functions inside the lambda